(suggestion) Limit max quantity of each engine

[tablesix]

Some rockets suggested will include stages with twinboar/mammoth engines, followed by another stage. This is very difficult to build. If users were able to limit the quantity of a certain part that were allowed in a build, this could be made less of an issue.

Allowing limitations on quantities of specific parts could also reduce the number of possible combinations, and possibly allow for a faster optimized rocket.